Illustration showing what the hot rocky exoplanet TRAPPIST-1 b could look like. TRAPPIST-1 b, the innermost of seven known planets in the TRAPPIST-1 system, orbits its star at a distance of 0.011 AU, completing one circuit in just 1.51 Earth-days. TRAPPIST-1 b is slightly larger than Earth, but has around the same density, which indicates that it must have a rocky composition. Webb’s measurement of mid-infrared light given off by TRAPPIST-1 b suggests that the planet does not have any substantial atmosphere. The star, TRAPPIST-1, is an ultracool red dwarf (M dwarf) with a temperature of only 2566 K and a mass just 0.09 times the mass of the Sun.
This illustration is based on new data gathered by Webb’s Mid-Infrared Instrument (MIRI) as well as previous observations from other ground- and space-based telescopes. Webb has not captured any images of the planet.
The 15-micron dayside brightness of TRAPPIST-1 b corresponds to approximately 500 K (230°C), which aligns with predictions for a tidally locked planet with a dark surface, no atmosphere, and no heat redistribution [1]. With an even heat distribution (e.g., due to a carbon dioxide-free atmosphere), the temperature drops to 400 K (125°C) [1]. A substantial amount of carbon dioxide in the atmosphere would result in even less emitted 15-micron light and cooler temperatures.
Although TRAPPIST-1 b's temperature is higher than Earth's, it is lower than Mercury's dayside, which has no significant atmosphere and receives 1.6 times more energy from the Sun [1]. The MIRI instrument, used in this study, was developed through a partnership between the European Space Agency (ESA), a European consortium of nationally funded institutes, the Jet Propulsion Laboratory (JPL), and the University of Arizona.
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